1. Field of the Invention
The present invention relates to a steering wheel for a motor vehicle and, more particularly, to a steering wheel unit including an air bag module.
2. Description of the Related Art
An air bag module for a conventional steering wheel unit of the type having four spokes fastened to spoke portions of its metal core, is described in U.S. Pat. No. 3,819,205. Although the plane P on which the rim R of the steering wheel extends as shown in FIG. 4, it is desireable for the top of the inflated air bag 12 of the module to extend substantially vertically at the time of actuation so as to maximize the cushioning action of the bag. It is also desireable for the inflated air bag 12 to be controlled so as to move backward, not forward (toward the driver), while rotating about the axis of the boss B of the steering wheel.
For that reason, it is desireable for the rigidity of the anterior spoke portions 3s(f) of the metal core of the wheel, which support the ring R at the front portion thereof, to be relatively high and for the rigidity of the posterior spoke portion 3s(r) which support the ring R at the wheel portion thereof to be relatively low, to appropriately determine the rigidity of the ring, which supports the inflated air bag 12 along nearly the entire circumference of the ring.
The air bag is normally folded in the module. A cover made of a soft synthetic resin covers the folded air bag and an inflater is provided for supplying a prescribed gas into the bag to inflate it. A box-shaped holder made of a metal plate holds the bag, the cover, and the inflater. The holder is sufficiently rigid that it does not deform when the air bag is inflated. Since the air bag module is fastened to the anterior and posterior spoke portions of the metal core of the steering wheel, the rigidity of those portions is reinforced, making it difficult to plastically deform those portions, particularly the posterior portion whose rigidity is desirably relatively low.
The above-mentioned conventional air bag module is fastened to the steering wheel of the unit as described in U.S. Pat. No. 3,819,205. The air bag module includes fastening portions which extend downwardly from the body of the module and have fastening holes substantially vertically penetrating the body of the fastening portions. The spoke portions of the metal core of the steering wheel have fastening holes extending substantially vertically therethrough and corresponding to the fastening holes of the fastening portions of the modules. The module is then fastened to the wheel with bolts which are fitted into the fastening holes of each of the module and the core.
Because the fastening holes extend through the fastening portions of the air bag module and the spoke portions of the metal core of the steering wheel, it is difficult to insert the bolts through the holes to fasten the module to the wheel, because of interference by the module. For that reason, it would be desireable to provide fastening holes which extend substantially horizontally, rather than vertically. Furthermore, since a strong force acts vertically in the axial direction of the ring of the steering wheel when the air bag module is actuated, it would also be desireable for the fastening holes to extend substantially horizontally, from the standpoint of the strength of the unit.
It is generally not problematic for such fastening portions to have substantially horizontal fastening holes. However if the metal core is manufactured from an aluminum alloy, a magnesium alloy, or the like by die-casting and the fastening holes are provided so as to extend substantially horizontally, it is necessary to use a slide core for die-casting, or to drill the holes after casting.
Indeed, during die-casting, dies are opened from and closed against each other in the axial direction of the ring of the metal core, which is substantially perpendicular to the direction of horizontal fastening holes. For that reason, if the metal core is manufactured by die-casting, and fastening holes are provided so as to penetrate substantially horizontally, it is necessary to use a slide core for casting or to drill the holes after casting. Manufacturing the metal core in this manner may take a great deal of time and is difficult.